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Mix Design Fundamentals
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In general, weight and volume terms are abbreviated as:
Xyz
(for instance, Gmb) |
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| Where: | X: | G | = | gravity |
| V | = | volume | ||
| W | = | weight | ||
| P | = | percentage | ||
| y: | b |
= | binder | |
s |
= | stone (aggregate) | ||
m |
= | mixture | ||
| z: | b |
= | bulk | |
e |
= | effective | ||
a |
= | apparent | ||
m |
= | maximum | ||
For example, Gmm = gravity, mixture, maximum: the maximum specific gravity of the HMA mixture. Figure 3 shows a complete list of terms.
Similar to soils, key HMA volumes can be expressed in a schematic diagram that clearly shows their relationship to one another. Figure 4 shows this schematic as well as what the volumes might look like in the HMA.
There are many different mathematical relationships between the various HMA, binder and aggregate properties. The more common ones are presented below.
The ratio of the mass in air of a unit volume of a permeable material (including both permeable and impermeable voids normal to the material) at a stated temperature to the mass in air (of equal density) of an equal volume of gas-free distilled water at a stated temperature. This value is used to determine weight per unit volume of the compacted mixture. It is very important to measure Gmb as accurately as possible. Since it is used to convert weight measurements to volumes, any small errors in Gmb will be reflected in significant volume errors, which may go undetected. See the HMA test to measure Gmb.
The ratio of the mass of a given volume of HMA without air voids (Va = 0) at a stated temperature (usually 25 °C) to a mass of an equal volume of gas-free distilled water at the same temperature. It is also called Rice Specific Gravity (after James Rice who developed the test procedure). Multiplying Gmm by the unit weight of water gives Theoretical Maximum Density (TMD). See the HMA test to measure Gmm.
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The total volume of the small pockets of air between the coated aggregate particles throughout a compacted paving mixture, expressed as a percent of the bulk volume of the compacted paving mixture. The amount of air voids in a mixture is extremely important and closely related to stability and durability. For typical dense-graded mixes with 0.5 inch (12.5 mm) nominal maximum aggregate sizes air voids below about 3 percent result in an unstable mixture while air voids above about 8 percent result in a water-permeable mixture.
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The volume of intergranular void space between the aggregate particles of a compacted paving mixture that includes the air voids and the effective asphalt content, expressed as a percent of the total volume of the specimen. When VMA is too low, there is not enough room in the mixture to add sufficient asphalt binder to adequately coat the individual aggregate particles. Also, mixes with a low VMA are more sensitive to small changes in asphalt binder content. Excessive VMA will cause an unacceptably low mixture stability (Roberts et al., 1996). Generally, a minimum VMA is specified and a maximum VMA may or may not be specified.
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The portion of the voids in the mineral aggregate that contain asphalt binder. This represents the volume of the effective asphalt content. It can also be described as the percent of the volume of the VMA that is filled with asphalt cement. For a constant VMA, VFA is inversely related to air voids: as air voids decrease, the VFA increases.
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The total asphalt binder content of the HMA less the portion of asphalt binder that is lost by absorption into the aggregate.
The volume of asphalt binder in the HMA that has been absorbed into the pore structure of the aggregate. It is the volume of the asphalt binder in the HMA that is not accounted for by the effective asphalt content.
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